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Abstract

Gypsum (CaSO4·2H2O) exposures and gypseous soils occupy over 100 million ha worldwide, primarily in arid and semiarid regions, with particularly large areas of surface gypsum in southwestern Asia, the Mediterranean region, the Horn of Africa and southwestern North America. Each of these areas hosts a diverse assemblage of gypsum endemic plant taxa, known as gypsophiles. Although plant biologists have been interested in the causes of gypsophily for well over a century, it has only been over the past few decades that gypsophile floras have received sustained ecological and evolutionary study. Recent work, principally in Spain, has revealed that both physical (e.g., gypsum crusts, soil porosity) and chemical (e.g., high Ca and S, low cation exchange capacity) factors may control community structure on highly gypseous substrates. Plant-fungal interactions may also play a key role in plant establishment on gypsum, although few studies have examined this subject. Molecular systematic and population genetic studies over the past two decades have revealed several key similarities in the assembly and evolution of gypsophile floras and taxa. These studies imply that gypsophile lineages have frequently appeared multiple times within clades that are ancestrally tolerant of gypsum, that speciation has been common in the most widespread lineages of gypsophiles, and that most gypsophile lineages first appeared no earlier than the latest Miocene. Population genetic studies have revealed generally higher levels of among-population genetic differentiation and isolation-by-distance within gypsophile taxa, in line with expectations for taxa that are restricted to substrate archipelagoes such as gypsum. Despite these advances in our understanding of gypsophily, gypsum floras remain much more poorly studied compared to other important edaphic endemic communities, such as serpentine and halophilic floras, highlighting the need for additional work.